1. Technical Field
The present invention relates to a process for producing maraging steel suitable for steel belts for continuous variable transmissions, and specifically relates to a technology for providing large residual compressive stress in a material.
2. Background Arts
A steel belt such as that mentioned above is wound around a pulley, and is traveled at high speed; the steel belt is therefore required to have high wear resistance and high fatigue strength to withstand the traveling and bending. As materials for such steel belts, maraging steels have been used in recent years.
Maraging steel is a super-high-tension steel with a high nickel content and has high tensile strength and high toughness due to a supersaturated martensite solid solution in which alloy elements are dissolved through a solution treatment followed by aging. In the past, maraging steel has been used in dies, and recently it has attracted attention due to the high tensile strength thereof, and it has therefore been used in steel belts, such as that mentioned above.
However, maraging steel does not have sufficient fatigue strength. Therefore, when maraging steel is used for an application in which high bending stress is applied, nitriding is performed on a thin plate made from maraging steel, thereby providing residual compressive stress in the surface portion and increasing fatigue strength. As a nitriding method, Japanese Patent Application, Second Publication, No. 116585/95 discloses a gas nitriding method in which a thin plate is heated in an atmosphere of pure ammonia gas as an aging treatment. However, maraging steel is difficult to nitrify since an oxide film readily forms on the surface thereof. Therefore, there is a disadvantage in that the processing time must be lengthened in order to obtain the desired residual compressive stress.
Japanese Patent Application, Second Publication, No. 82452/93 discloses a method in which material is bent after a solution treatment to obtain residual compressive stress and is subjected to ammonia gas nitriding as aging treatment. The publication notes that the method can promote nitriding by providing residual compressive stress before the nitriding processing, and can increase surface hardness and residual compressive stress. However, it has been demonstrated that the residual stress provided before the nitriding processing is relaxed by the nitriding processing, and the required residual compressive stress cannot be obtained by this method. Moreover, control of the nitriding processing is difficult since it occurs rapidly, and therefore the effects of the nitriding processing varies greatly. There was also a problem in that the quality thereof varied from batch to batch.
Japanese Patent Application, First Publication, No. 154834/90 proposes a method in which material is subjected to an ammonia gas nitriding processing after aging and is then shot-peened. The publication notes that the duration for nitriding can be controlled, and therefore the desired residual compressive stress can be reliably obtained.
However, the method in Japanese Patent Application, First Publication, No. 154834/90 poses a problem in that the duration for nitriding processing is long and the producing cost is high since the method requires the additional process of shot-peening.
It is known that inclusions contained in steel belts greatly affect fatigue strength in high cycle fatigue tests, and that larger inclusions more can more readily initiate fatigue failure, thereby shortening the service life of the steel belts. FIG. 5 is a diagram showing the relationship between the frequency of the repeated bending load and the tension load applied to the steel belt when a fatigue failure occurred in a steel belt which was wound around two pulleys and is traveled. As shown in FIG. 5, in the low cycle side in which the frequencies of the repeated bending load are 10.sup.5 or less, fatigue failures initiated at the surfaces of the steel belts. In contrast, in the high cycle side in which the frequency of the repeated bending load is 10.sup.7 or more, fatigue failures initiated at the inclusions in the steel belts. As steel belts for CVT are used in higher cycle frequencies of repeated bending load, and it is therefore understood that it is very important to reduce the size and number of inclusions in order to ensure sufficient fatigue strength to withstand the traveling and bending.
As methods of measuring inclusions, there may be, for example the United States standards ASTM: E1245-89 (measuring method for inclusions in steel and other metals by automatic image analysis) and ASTM: E1122-92 (evaluation method for jk inclusions by automatic image analysis), and these method are similar to methods used in other countries. A method may also be mentioned in which the proportion of the number of inclusions on the standard lattice points provided in a visual field of a micro-photograph or a video camera, which is used in Japan.
However, in the above methods, as a section exposed on a surface of a sample is measured, the actual size is typically larger than the result of the measurement. Therefore, in evaluation methods for inclusions in maraging steel for steel belts which concern large inclusions, the correlation between the evaluation result and fatigue strength is low, and there is therefore a problem in reliability. Recently, statistics extremes methods in which the maximum size of inclusions is estimated based on the size of one section of an inclusion has attracted attention (for example as in, Anti corrosion Engineering, Vol. 37, pages 768 to 773 (1988); Japanese Patent Application, First Publication, No. 2073/94; and Japanese Patent Application, First Publication, No. 170502/98). Generally, it is assumed that the distribution of inclusions in metallic material is similar to an exponential distribution. Furthermore, it is known that the extremes distribution seems to follow a double exponential distribution, and therefore the maximum size of inclusions can be estimated by using a statistics extremes method. In the following, the process for evaluating inclusions by a general statistics extremes method is shown.
(1) Extraction of Sample
A sample is cut along a face perpendicular to the direction of principal stress, and the sample surface is ultimately polished by using #2000 sandpaper, and is then finished by buffing to a specular surface.
(2) Image Processing of Inclusions
The sample surface is photographed by microphotography or by a video camera, and one visual field obtained thereby is defined as an standard inspecting area, and the inclusion with the maximum area is specified in the standard inspecting area. The square root ( (area)) of the area of the inclusion with the maximum area is calculated, and such a procedure is repeated N times in such a way that the inspection portions (visual fields) do not overlap.
(3) Statistical Processing
As shown in FIG. 6, the square root ( (area)) of the area is plotted on a statistics extremes sheet. Then, a straight line is applied to the plotted points, and the value of the X coordinate is estimated as being the maximum size of the inclusions when the line is extrapolated to the recurrent period T.
However, in the measurement method for inclusions using the statistics extremes method, the object for measurement is a section of an inclusion exposed on the surface of the sample, and the actual size of the inclusion is not directly measured, but is merely estimated. It is therefore difficult to precisely measure the size of the inclusion using such methods. As a result, in conventional measurement methods, a very high safety margin must be set for the material strength in consideration of the effects of the inclusions on fatigue strength.
In order to improve wear resistance and fatigue strength to withstand the traveling and bending of steel belts and the like, method have been adopted in recent years in which the effects of elements, such as carbon and nitrogen, contributing to the formation of inclusions, have decreased. In particular, high-purity maraging steel can be produced by the methods in which nitride inclusions, typified by TiN, and carbide inclusions, typified by TiC, are not formed, and therefore very few inclusions exist in the maraging steel. As a result, evaluation of inclusions according to the sampling based on the present statistics extremes method varies greatly since the proportion of fine inclusions is large, and therefore the accuracy in statistical processions is low and the reliability is insufficient; so that the selection accuracy for the product is therefore insufficient.